The present invention relates to a rolling bearing structured such that a rotary body with an outer ring fitted therewith and a shaft with an inner ring fitted therewith can be connected together by a clutch mechanism and, when the rotary body and shaft are connected together by the clutch mechanism, with no relative rotation between the inner and outer rings, the rolling bearing can be used on receiving a rotation load; and, in more particular, to an improvement in such rolling bearing which can prevent occurrence of fretting in the bearing raceway surfaces thereof to thereby enhance the life thereof.
Now, FIGS. 1 and 2 show the states of use of a rolling bearing.
A rolling bearing 1 shown in FIGS. 1 and 2 is a combined bearing, in which, between the raceway surface 2a of an outer ring 2 and the raceway surface 3a of an inner ring 3, there is interposed spherical-shaped rolling elements 4. The rolling elements 4 are respectively held by a retainer (not shown) in such a manner that they are spaced at given intervals in the peripheral direction of the raceway surfaces 2a, 3a. Also, grease is sealed in by a seal (not shown).
In the rolling bearing 1, the outer ring 2 is fitted with the inner periphery of a rotary body 6, whereas the inner ring 3 is fitted with the outer periphery of a shaft 8. The rotary body 6 is a pulley which can be driven through a belt wound on a belt groove 6a formed in the outer periphery thereof, while the other end portion (not shown) of the rotary body 6 provides an output end thereof.
The rotary body 6 and shaft 8 can be connected together by a clutch mechanism 12.
The clutch mechanism 12 is an electromagnetic clutch which is composed of a clutch plate 13 fixed to the end portion of the shaft 8 and an electromagnetic drive portion 14 for attracting the outer peripheral portion of the clutch plate 13 to the rotary body 6 using an electromagnetic force. And, FIG. 1 shows a clutch-off state in which the rotary body 6 and shaft 8 can be rotated with respect to each other, whereas FIG. 2 shows a clutch-on state in which the rotary body 6a nd shaft 8 is prevented against their relative rotation.
In the clutch-off state shown in FIG. 1, a rotation force input to the rotary body 6 is not transmitted to the shaft 8, but the outer and inner rings 2 and 3 of the rolling bearing 1 can be rotated with respect to each other. On the other hand, in the clutch-on state shown in FIG. 2, since the rotary body 6 and shaft 8 is rotated integrally due to the rotary force input to the rotary body 6, the relative rotation between the outer and inner rings 2 and 3 of the rolling bearing 1 is zero.
By the way, generally, when designing a rolling bearing, in order to reduce noise in a high speed operation and to prevent play between the rotary body 6 and shaft 8, an initial radial clearance between inner and outer rings is set in such a manner that a bearing effective clearance when the rolling bearing is incorporated into between the rotary body 6 and shaft 8 can be reduced as much as possible.
However, as shown in FIG. 2, in the clutch-on state, when the outer and inner rings 2 and 3 are driven or rotated with their relative rotation zero, as shown by arrow marks A-E in FIGS. 7(a)-(e), with the rotation of the outer and inner rings 2 and 3, the direction of a rotation load is moved. And, when the rolling body 4 is situated directly below the rotation load direction, a load acting on the rolling body 4 increases; and, therefore, as shown by solid lines in FIGS. 8(a) and (b), a contact area S1 between the rolling body 4 and raceway surface 3a increases. On the other hand, when the rolling body 4 is situated on the opposite side to the rotation load direction, the load acting on the rolling body 4 decreases; and, therefore, as shown by broken lines in FIGS. 8(a) and (b), a contact area S2 between the rolling body 4 and raceway surface 3a decreases.
And, as described above, especially, in case where the bearing effective clearance is set small, that is, for example, the bearing effective clearance is zero, contact positions between the respective rolling elements 4 and raceway surface 3a remain unchanged. Therefore, variations in the above-mentioned load acting on the rolling body 4 cause slight slippage between the mutual contact surfaces of the raceway surface 3a and rolling body 4 and thus, in case where the rolling body 4 collides repetitively with the raceway surface 3a at the same position, there occurs fretting which gives rise to local wear on the raceway surface 3a. This local wear causes strange sounds and reduces the life of the bearing. Or, due to worn powder produced in the local wear, the life of the grease can be shortened.
In order to prevent the local wear which is caused by such fretting, conventionally, there is also proposed a technique in which a coating film is formed on the surface of the rolling body 4 or on the raceway surface 3a to thereby prevent the raceway surface 3a and rolling elements 4 against direct contact with each other (see Japanese Patent Unexamined Publication No. Hei.11-218134). However, in this technique, formation of the coating film complicates a process for manufacturing the component parts of a rolling bearing, which results in the increased cost of the rolling bearing.